ELECTRON-SPIN-RESONANCE IN THE DOPED SPIN-PEIERLS COMPOUND CU1-XNIXGEO3

Electron spin-resonance studies of the Ni-doped spin-Peierls compound CuGeO3 have been performed for the frequency range 9-75 GHz and temper ature interval 1.3-20 K. An anomalous temperature dependence of the g- factor below the spin-Peierls temperature was observed for doped sampl es. At low temperatures the g-factor is much smaller than the value ex pected for Cu2+ and Ni2+ ions and is much more anisotropic than for an undoped crystal. This anomaly is explained by the formation of magnet ic clusters around the Ni2+ ions within a nonmagnetic spin-Peierls mat rix. The formation of magnetic clusters is confirmed by the observatio n of a nonlinear static magnetic susceptibility at low temperatures. T he reduction of the spin-Peierls transition temperature was found to b e linear in the dopant concentration x over the range 0 less than or e qual to x less than or equal to 3.2%. The transition into the antiferr omagnetically ordered state, detected earlier by neutron scattering fo r x greater than or equal to 1.7%, was studied by means of ESR. For x = 3.2% a gap in the magnetic resonance spectrum is found below the Nee l temperature and the spectrum is well described by the theory of anti ferromagnetic resonance based on the molecular-field approximation. Fo r x = 1.7% the spectrum below the Neel point remained gapless. The gap less spectrum of the antiferromagnetic state in weakly doped samples i s attributed to the small value of the Neel order parameter and to the magnetically disordered spin-Peierls background.